This invention relates to a special cannula that is usable during heart by-pass surgery and where the cannula is capable of being employed to remove blood directly from the heart while at the same time, provide a device to continuously monitor the pressure within the heart during that blood removal.
In carrying out various procedures that require a heart bypass operation, blood is removed directly from the patient's heart and to do so, a cannula is introduced into the left side of the heart i.e. from the left ventricle or atrium where the blood is pumped away and then reintroduced into the heart through the right side of the heart, typically via the ascending aorta. The blood is pumped from the heart by means of a special pump, one of which is the Jarvik heart pump and which pulls the blood from the left heart and replaces the blood back into the right heart. A cannula is therefore placed with its distal end into the particular chamber of the patient's heart and which sucks the blood from that chamber. The cannula is generally a very large device to be capable of relatively large flows of blood and, as an example, can be of the size of 24 to 28 French. Since the cannula is directly introduced into the heart to withdraw the blood, it is obviously of extreme importance that the pressure in the heart chamber not become negative, that is, where the heart chamber could collapse from the over-removal of the blood. The danger is that the blood will be removed at such a high rate of flow that the heart itself will be sucked dry. Accordingly, in heart bypass procedures, a pressure transducer is used to maintain an internal constant monitor of the pressure of the blood within the heart chamber as it is being removed. As such, therefore, the pressure transducer can even, if desired, have control of the pump and slow down the pump in the event the pressure within the heart chamber is reduced to a predetermined pressure, approaching a negative value, and which assures that the negative pressure is not reached within the heart.
In current devices, a liquid filled column is used and which is introduced into the heart adjacent the location of the distal end of the cannula that is removing the blood. There are, however, several drawbacks to the use of a liquid filled column in determining the pressure of the blood in the heart chamber. As one difficulty, the tip of the liquid filled catheter or tube containing the liquid can become occluded if it presses against the inner heart wall and therefore will not read the correct pressure in the heart. In addition, the transducer that is attached to the catheter or tube and which is external of the patient, must be positioned at the heart level at all times. Since the pressures being measured in the heart are so low, the error that will be caused by not having the transducer at the exact level can be critical, that is, a difference of one inch in position of the transducer below the level of the heart will indicate 2 mmHg. higher than the actual pressure and such a difference is considerable when taking into account that the pressures measured in the heart may approach 0 mmHg. and, as stated, it its critical that the pressure remain positive.
One type of pressure sensor that has been used in a patient to monitor pressure, for example, is the pressure tip cannula where an electrical sensor, in the form of a electronic chip, is located at the distal end of the catheter or cannula and which is positioned at the location where the pressure is desired to be determined. In such sensors, the chip is a piezoresistive crystal that is positioned between the chamber where the pressure is being measured and a chamber having a know reference pressure, such as atmospheric pressure. As the chip flexes in response to changes in pressure, its resistance changes and that change in resistance is sensed by a Wheatstone bridge and a electronic instrument converts the changes in resistance into measurements of pressure. Such devices have been used for a variety of purposes, such as intrauterine pressure transducers and the like, however the use of such a transducer within a chamber in the patient's heart during a bypass operation requires extreme accuracy since the pressures involved are extremely low, i.e. a few mmHg. In addition, it is critical that the pressure in the chamber of the heart where the blood is being withdrawn not be negative. Accordingly, as the cannula is sucking blood from the interior of the patient's heart, it is necessary to deal with extremely low pressures since there is a real danger in the event the pressure becomes negative in the chamber where the blood is being pumped out yet the accuracy must be extremely precise due to the critical nature of the operation and the well being of the patient.
One problem associated with the use of pressure tip transducers is that they can be affected by the moving flow of liquid moving past the transducer itself. As previously explained, due to the low pressures being measured and the need for extremely high accuracy, any affect that could alter the accuracy of the instrument is critical. With the use of a large cannula that is withdrawing blood at a fairly large flow, therefore, it is important that the piezoresistive chip transducer not be affected by the dynamics of the flow. For example, if the pressure sensitive side of the chip is exposed to the flowing blood, the blood tends to draw the chip in the direction of flow and therefore the pressure readings may be less than the actual pressure that is present within the heart chamber. In addition, it is important that the chip itself not block or in any way hinder the flow of the blood as it is being withdrawn from the heart and therefore must be positioned in a special location to avoid such problem. The use of a chip in such a situation also raises the problem that the piezoresistive chip must be operable even when contacting one of the internal walls of the heart.
Another possible source of inaccuracies in such pressure tip monitor cannulas is the electrical sensor, located at the distal end of the cannula, has an inherent temperature drift and which causes a zero drift change due to a temperature change, that is, as the temperature changes, the sensor will experience a drift in its zero reading. While the presence of that drift may be acceptable when dealing with the higher pressures involved in the normal blood pressure of a patient, the same temperature drift can introduce an unacceptable inaccuracy into the pressure monitoring system at the range of pressures encountered in the heart chamber during the bypass operation.